We have further explored the molecular mechanisms that induce tyrosine hydroxylase (tyrosine-3-monooxygenase) transsynaptically in rat adrenal medulla. The present investigation has emphasized the detailed mechanisms of the uptake by nuclei of the catalytic subunits of cytosol ATP: protein phosphotransferase (protein kinase, PK). We have provided evidence that the catalytic subunits of cytosol PK can be taken up into isolated nuclei following dissociation of the holoenzyme by cAMP in vitro. These newly taken up catalytic subunits of PK are tightly bound to chromatin and phosphorylate primarily the nonhistone proteins of chromatin. RNA synthesis in isolated nuclei was significantly enhanced following the uptake of PK and enhanced phosphorylation of nuclear proteins. The PK's endogenously present in medullary nuclei were also isolated and characterized. BIBLIOGRAPHIC REFERENCES: Chaung, D.M. and Costa, E.: Trans-synaptic regulation of ribonucleic acid biosynthesis in rat adrenal medulla. Mol. Pharmacol. 12: 514-518, 1976. Costa, E., Chuang, D.M., Guidotti, A. and Hollenbeck, R.: Control of nuclear function in chromaffin cells by persistent activation of nicotinic receptors. In Jenden, D.J. (Ed.): Cholinergic Mechanisms and Psychopharmacology. New York, Plenum Press, 1977.